Sacabambaspis, a 460-million-year-old jawless relative of modern jawed fishes, swims in the shallow coastal waters of modern day BoliviaCREDIT: Nobumichi Tamura

Evolution of the major groups of fish that we recognize today took place in shallow waters, close to the seashore, according to new research at the University of Birmingham.

The findings, published in Science, suggest that while coral reefs may be vital for diversification in present day, fragile near-shore environments were crucial for evolution some 480-360 million years ago.

These results also place a new emphasis on the vulnerable nature of the near-shore under the impacts of climate change and the importance of these areas in generating future diversity.

Researchers in the University of Birmingham's School of Geography, Earth and Environmental Sciences, working with colleagues at the Universities of Pennsylvania, Michigan, and Manchester, surveyed fossil records of primitive fish across the globe.

Their aim was to complete a missing link in our evolutionary story. Although we know there was a huge proliferation of fish species present by around 420 million years ago, fossil records from before this time are hard to find. These gaps have made it difficult to build a convincing picture of early diversification.

To address this challenge, the team took a "big data" approach, surveying more than 2,700 early records and using mathematical modeling to predict the likely habitat type from where these early fish types emerged.

The team was able to clearly identify shallow, near-shore waters where primitive, jawless fish made their first appearance. Fish that developed more flexible body shapes then moved out from the shore and into deeper waters. Some fish groups moved into freshwater, while others evolved into early land-dwelling vertebrates.

"Most of today's major groups of fishes, from jawless hagfish and lampreys through to sharks and the boney fish we are familiar with, make their first appearance during the Ordovician, around 480 million years ago, or shortly thereafter" says Dr Ivan Sansom, senior lecturer in palaeobiology at the University of Birmingham and co-lead author of the study. "The big surprise for us was that all of these seem to have started their evolutionary journey in these very shallow waters close to the shoreline."

Movement of these fish into deeper water was restricted to groups with flexible body shape, which were able to use their increased mobility to move into deeper waters and become established over the following 20 million years—throughout the Silurian and Devonian periods.

The survey showed that more heavily armored, rigid fish would have had limited swimming ability, and would not have managed to disperse into deeper waters.

"One reason why these fossils have been hard to study is precisely because of the area in which they are found. Wave action in the shallow ocean area likely blasted them into tiny fragments," says professor Lauren Sallan, a palaeobiologist at the University of Pennsylvania and co-lead author of the study.

The team realized that the rocks in which many of the earliest fish were preserved were all formed in extremely similar environments, suggesting there were as yet unknown features in these environments that may be particularly advantageous in driving fish evolution.

"We don't yet know exactly what was special about those early, shallow marine environments that enabled earliest fish to take those first evolutionary steps, but we do think they provided evolutionary 'hotspots,' responsible for generating this rich diversity," says Dr Sansom.

He added: "This work highlights how important these increasingly vulnerable near shore areas are for species evolution. Modern threats from a combination of climate change, elevated sea levels, over-fishing and pollution could have extremely damaging effects on future species diversification."

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